1. Field of the Invention
This invention relates to an optical filtering device. More particularly, although not exclusively, it relates to a device suitable for selectively reducing the intensity of unwanted radiation in an optical system, such as that arising from a high intensity monochromatic localised source.
2. Discussion of Prior Art
Dazzle produced by high intensity radiation sources is a common problem in optical systems such as imaging devices. United Kingdom Patent No. 1,115,864 describes a gas cell used to protect a radiation detector against damage from high intensity radiation. The pressure of gas in the cell is such that, at sufficiently high radiation intensity, the gas breaks down into a plasma which scatters and attenuates unwanted radiation. However, such an arrangement provides no improvement in imaging performance in the presence of a dazzling source. It merely provides protection against damage. There is for example a need for a device capable of selectively blocking radiation from a high intensity source, while allowing other radiation from a scene containing such a source to be collected for use in imaging. Optical apparatus having a need for such a device includes television and film cameras, binoculars, night sights, flying helmets, safety goggles and optical instruments incorporating sensitive detectors.